Hello Chardiver!
We have recently had a question similar to yours on our web site, Diving Medicine Online, and none of our consultants could come up with a consistently reasonable answer. (If this was you, please forgive me!)
The best answer that I could find was in relationship to the effects of immersion, which has many widespread effects on the body - the most noticeable being the diuresis of diving (the pee factor). This is a complicated feedback mechanism that is started by a shift of body fluids centrally, stretching some receptors in the heart. This in turn sets off all sorts of neuroendocrine hormonal reactions.The basic response to any sort of immersion is a redistribution of blood volume.The intrathoracic blood volume expansion is
associated with an increased release of the Atrial Natriuretic Peptide (ANP) and an alteration of the activity of the posterior pituitary gland.
Studies also suggest that hormones of anterior pituitary origin are required for ANP secretion in response to acute volume loading.
To make a long story short - the possible cause of your breast engorgement may be due to stimulation of your anterior pituitary gland with the production of hormones that act on certain end organs in the body - in your case, your breasts. This is somewhat analagous to the premenstrual engorgement of the breasts that so many women experience. Why this happens to you and not to other women is the question and might cause you to want to check in with your physician with the problem.
You must understand that this is all supposition, as we could not come up with a better answer to your question.
Best regards for safe diving!
scubadoc
Diving Medicine Online
The Ten Foot Stop (Newsletter for all divers)